1. Field of the Invention
The present invention relates to a data communications transceiver used for base band packet switching, such as an Ethernet system.
2. Description of the Prior Art
A transceiver such as is used with an Ethernet system is generally disclosed in U.S. Pat. No. 4,317,205 of Tat C. Lam for a "Wideband Transceiver With EMI Suppression", granted Feb. 23, 1982.
In conventional data communication transceivers, the transmitter section is either capacitor coupled or transformer coupled to the communicating medium which medium is most commonly a shielded coaxial cable. Both methods of coupling use a regulated voltage source or emitter-coupled logic circuitry to drive the coaxial cable as found, for example, in U.S. Pat. No. 4,412,347 of Jesse B. Lipcon entitled "Precision Setting of Currents and Reference Voltages", granted Oct. 25, 1983. The voltage level of the signal will affect the skew of the received data. Noise or reflection on the transmission line can be coupled to the receiver which in turn will be received as an unwanted signal. Furthermore, the capacitive AC coupling will limit the low frequency data to the extent of the differentiation being used by the coupling.
Another problem is that when the transceiver is transmitting, electromagnetic interference may be present around the connection between the connector to the shielded cable and the shielded container of the transceiver. This electromagnetic interference is believed to be due to stray capacitance between the center conductor of the connector and the shielded container. A conventional way to minimize this effect is to shield the exposed center conductor or to make the outer connector much more capacitive relative to the shielded container than to the center conductor. This can produce switching spikes, which cause unwanted interference in the atmosphere around the transceiver because of high speed switching and this AC coupling to ground. Still another problem occurs where the transmission medium is DC coupled. The capacitance effect of the transmitter is passed to the coaxial cable, loading the communicating medium. This is found in U.S. Pat. No. 4,384,363 of Jesse B. Lipcon entitled "Transceiver For Local Network Using Carrier-Sense Multiple Access/Collision Detection", granted May 17, 1983.